Process for low-temperature separation of air

ABSTRACT

In a low-temperature separation of air in a distillation system, having at least one high-pressure column ( 11 ) and one low-pressure column ( 12 ), process air ( 20, 32 ) is introduced into the high-pressure column ( 11 ). A liquid product stream ( 42; 46 ) is removed from the distillation system, brought in the liquid state to an elevated pressure ( 43; 47 ), evaporated or pseudo-evaporated under this elevated pressure by indirect heat exchange ( 6 ), and finally drawn off as a gaseous product stream ( 45; 49 ). All of the process air ( 1 ) is compressed in a main air compressor to a first pressure, which is at least equal to the operating pressure of the high-pressure column ( 11 ) and then is purified in a purification device. A first partial stream ( 2, 7 ) of the process air is fed under approximately the first pressure to a first expander ( 8 ), actively depressurized there to approximately the operating pressure of the low-pressure column ( 12 ), and then introduced ( 10 ) into the low-pressure column ( 12 ). A second partial stream ( 3 ) of the process air is compressed in a first secondary compressor ( 14 ) to a second pressure that is higher than the first pressure. At least one portion ( 17 ) of the second partial stream ( 16 ) downstream from the compression is further compressed in a second secondary compressor ( 18 ) to a third pressure, which is higher than the second pressure, fed to the indirect heat exchange ( 6 ) for evaporation or pseudo-evaporation of the liquid product stream, and then introduced ( 20 ) into the distilling-column system. The second secondary compressor ( 18 ) is designed as a cold compressor. At least a portion of the mechanical energy that is produced in the active depressurization ( 8 ) of the first partial stream ( 7 ) is used to drive the second secondary compressor ( 18 ).

The invention relates to a process by low-temperature separation of air, in particular for producing gaseous compressed oxygen.

Processes and devices for low-temperature separation of air are known from, for example, Hausen/Linde, Tieftemperaturtechnik [Low-Temperature Technology], 2nd Edition 1985, Chapter 4 (pages 281 to 337).

The distilling-column system of the invention can be designed as a two-column system (for example as a standard Linde double-column system) or else as a three-column or multiple-column system. In addition to the columns for nitrogen-oxygen separation, additional devices can be provided to recover other air components, in particular noble gases, for example an argon or a krypton-xenon recovery.

The invention relates in particular to a process in which at least one gaseous compressed product is recovered by a liquid product stream being removed from the distilling-column system for nitrogen-oxygen separation, brought to an elevated pressure in the liquid state, and evaporated under this elevated pressure by indirect heat exchange or pseudo-evaporated (at supercritical pressure). Such internal compression processes are known from, for example, DE 830805, DE 901542 (=U.S. Pat. No. 2,712,738/U.S. Pat. No. 2,784,572), DE 952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat. No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE 1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No. 3,222,878), DE 1199293, DE 1187248 (=U.S. Pat. No. 3,371,496), DE 1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat. No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 (=U.S. Pat. No. 3,500,651), DE 2535132 (=U.S. Pat. No. 4,279,631), DE 2646690, EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No. 5,036,672), EP 505812 B1 (=U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S. Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733 B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1(=U.S. Pat. No. 6,038,885), DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1(U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755), DE 19909744 A1, EP 1067345 A1 (U.S. Pat. No. 6,336,345), EP 1074805 A1 (=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1134525 A1 U.S. Pat. No. 6,477,860), DE 10013073 A1, EP 1139046 A1, EP 1146301 A1, EP 1150082 A1, EP 1213552 A1, DE 10115258 A1, EP 1284404 A1 (=US 2003051504 A1), EP 1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211 A1, EP 1357342 A1 or DE 10238282 A1.

One object of the invention is to provide such a process and a corresponding device in an especially advantageous manner economically. Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

The objects are achieved by a process for low-temperature separation of air in a distilling-column system that has at least one high-pressure column (11) and one low-pressure column (12), in which

-   -   Process air (20, 32, 132) is introduced into the high-pressure         column (11),     -   A liquid product stream (42; 46; 346) is removed from the         distilling-column system, brought in the liquid state to an         elevated pressure (43; 47; 347) and evaporated or         pseudo-evaporated under this elevated pressure by indirect heat         exchange (6), and finally drawn off as a gaseous product stream         (45; 49; 349),     -   All of the process air (1) is compressed in a main air         compressor to a first pressure, which is at least equal to the         operating pressure of the high-pressure column (11), preferably         to a pressure of more than 1 bar and particularly more than 3         bars higher than the operating pressure of the high pressure         column,     -   And then is purified in a purification device,     -   A first partial stream (2, 7) of the process air is fed under         approximately the first pressure to a first expander (8),         actively depressurized there to approximately the operating         pressure of the low-pressure column (12), and then introduced         (10) into the low-pressure column (12),     -   A second partial stream (3) of the process air is compressed in         a first secondary compressor (14) to a second pressure that is         higher than the first pressure, and     -   At least one portion (17) of the second partial stream (16)         downstream from the compression in a second secondary compressor         (18) is further compressed to a third pressure, which is higher         than the second pressure, fed to the indirect heat exchange (6)         for evaporation or pseudo-evaporation of the liquid product         stream, and then introduced into the distilling-column system         (20),     -   whereby the second secondary compressor (18) is designed as a         cold compressor,

characterized in that

-   -   At least a portion of the mechanical energy that is produced in         the active depressurization (8) of the first partial stream (7)         is used to drive the second secondary compressor (18).

Having the cold compressor be driven by the first expander is especially advantageous in terms of energy in particular. In many internal compression processes that are run at relatively high air pressure, the process air exhibits a pressure potential that would produce more cold in the expander(s) than can be used in the process. The excess energy is used in the invention to drive the cold compressor, which brings the second partial stream of the process air to an especially high pressure.

Preferably, the process has a second expander, in which a third partial stream of the process air is actively depressurized. The exhaust pressure of the second expander is, for example, approximately at the level of the low-pressure column or the high-pressure column. Depending on the case, the depressurized air is introduced into the low-pressure column or into the high-pressure column.

BRIEF DESCRIPTION OF DRAWINGS

The invention as well as additional details of the invention are explained in more detail below based on the embodiments that are depicted in the drawings. In this connection:

FIG. 1 shows a first embodiment of the process according to the invention with two blast turbines,

FIG. 2 shows a second embodiment, in which a turbine in the high-pressure column is depressurized,

FIG. 3 shows another embodiment with two blast turbines, and

FIG. 4 shows a fourth embodiment that combines aspects of FIGS. 2 and 3.

In FIG. 1, atmospheric air is compressed in a main air compressor to a first pressure of, for example, 5 to 7.5 bar, preferably 5.5 to 6 bar, and then purified in a purification device (not shown). The purified process air 1 is divided into four partial streams 2, 3, 4, 5 under, for example, the first pressure.

The first partial stream 2 is fed to the hot end of a main heat exchanger 6, cooled there to a first intermediate temperature, removed again via line 7, and actively depressurized in a first expander 8 to a pressure of, for example, 1.3 to 1.8 bar, preferably 1.3 to 1.6 bar. The actively depressurized first partial stream is introduced via the lines 9 and 10 into the low-pressure column 12 of a distilling-column system, which in addition has a high-pressure column 11 and a main condenser 13.

The second partial stream 3 of the process air is further compressed in a first secondary compressor 14 to a second pressure of, for example, 29 to 60 bar, preferably 35 to 50 bar, and after cooling, it flows into a secondary condenser 15 via line 16, also to the hot end of the main heat exchanger 6. At a second intermediate temperature, the second partial stream is removed via line 17 and is fed to a second secondary compressor 18, which is designed as a cold compressor and is coupled mechanically to the first expander 8. The exhaust pressure of the cold compressor 18 (“third pressure”) is, for example, 40 to 85 bar, preferably 45 to 70 bar. Via line 19, the high-pressure air is fed into the main heat exchanger 6 at a third intermediate temperature that is higher than the first intermediate temperature, and it flows through said exchanger up to the cold end. The third partial stream 19 under the third pressure is cooled in the main heat exchanger and condensed or pseudo-condensed (at supercritical pressure). The cold high-pressure air 20 is depressurized in a butterfly valve 21 to approximately the operating pressure of the high-pressure column 11, which is, for example, 5 to 7.5 bar, preferably 5.5 to 6 bar, and introduced into the high-pressure column. At least one portion of the introduced liquid air is removed again via line 22, cooled in a subcooling countercurrent device 23, and fed via line 24 and butterfly valve 25 into the low-pressure column 12.

The third partial stream 4 of the process air is further compressed in a third secondary compressor 26 with a secondary condenser 27 to a fourth pressure of, for example, 7.5 to 11 bar, preferably 8 to 9 bar, and it is conveyed via line 28 to the main heat exchanger 6. At a fourth intermediate temperature, the cooled third partial stream 29 is sent to a second expander 30 and actively depressurized there to a pressure of, for example, 1.3 to 1.8 bar, preferably 1.3 to 1.6 bar. The actively depressurized third partial stream 31 is sent together with the first partial stream 9 via line 10 to the low-pressure column 12. The second expander 30 is coupled mechanically to the third secondary compressor 26 and drives the latter. Both expanders are preferably designed as turboexpanders and depressurize to approximately the pressure of the low-pressure column (blast turbines).

The fourth partial stream 5 of the process air flows through under approximately the first pressure to the main heat exchanger 6 and is sent via line 32 in gaseous form to the bottom of the main pressure column 11.

Liquid crude oxygen 33 is cooled in the subcooling countercurrent device 23 and fed via line 34 and butterfly valve 35 into the low-pressure column 12. A portion of the top gas of the high-pressure column 11 is drawn off via line 36, heated in the main heat exchanger 6 to approximately ambient temperature, and finally drawn off at 37 as a gaseous medium-pressure nitrogen product. The remainder of the top gas is condensed in the main condenser 13. The liquid nitrogen 38 that is recovered in this case is released to a first portion 39 via the subcooling countercurrent device 23, a line 40 and a butterfly valve 41 as reflux to the top of the low-pressure column 12. A second portion is used as reflux in the high-pressure column 11. A third portion 42 is brought in a nitrogen pump 43 in liquid state to an elevated pressure of, for example, 10 to 50 bar, preferably 10 to 15 bar, conveyed via line 44 to the main heat exchanger 6 and evaporated or pseudo-evaporated there under this elevated pressure by indirect heat exchange with process air and heated to approximately ambient temperature. It leaves the unit via line 45 as a gaseous compressed nitrogen product.

From the bottom of the low-pressure column 12, liquid oxygen 46 is drawn off, brought in an oxygen pump 47 in the liquid state to an elevated pressure of, for example, 10 to 50 bar, preferably 12 to 40 bar, conveyed via line 48 to the main heat exchanger 6, and evaporated or pseudo-evaporated there under this elevated pressure by indirect heat exchange with process air and heated to approximately ambient temperature. It leaves the unit via line 49 as a gaseous compressed oxygen product.

Gaseous nitrogen 50 is drawn off from the top of the low-pressure column 12 and heated in the subcooling countercurrent device 23 and in the main heat exchanger 6. The hot, depressurized nitrogen 51 can be used as a product, discarded, and/or used in the unit as regeneration gas in the purification device, not shown, and/or as dry gas in an evaporative condenser for cooling water.

FIG. 2 is thus distinguished from FIG. 1 in that the exhaust pressure of the second expander 130 is higher; it is namely at the level of the operating pressure of the high-pressure column 11. Consequently, the actively depressurized third partial stream 131 is purified here with the cold fourth partial stream and is fed via line 132 to the high-pressure column 11.

In addition, in the process of FIG. 2, liquid products are recovered by a portion 136 of the liquid oxygen being drawn off from the bottom of the low-pressure column 12 as a liquid oxygen product (LOX) and a portion of the liquid nitrogen 142 a, 142 b produced in the main condenser 13 being drawn off as a liquid nitrogen product (LIN).

Reflux liquid 139-140-141 for the low-pressure column 12 is removed here from the high-pressure column 11 at an intermediate point.

The process of FIG. 3 is distinguished from that of FIG. 1 in that the first, second and third partial streams are further compressed first together (203) in a fourth secondary compressor 214 with a secondary condenser 215 to an intermediate pressure. (In this case, the secondary compressors 214 and 14 can be formed from different stages of the same machine). Only then is the stream under the intermediate pressure divided into the first partial stream 2, the second partial stream 3, and the fourth partial stream 4. As a result, higher inlet pressures are produced at the two turbines 8, 30, namely, for example, 12 to 16 bar, preferably 12 to 14 bar, on the first expander 8 (“second pressure”) and, for example, 12 to 16 bar, preferably 12 to 14 bar, on the second expander 30 (“intermediate pressure”). In this respect, a relatively large proportion of products can be recovered in liquid form. The removal of the liquid product is done analogously to FIG. 2 from the bottom of the low-pressure column 12 (LOX with a purity of 99.5% via lines 146 a, 146 b and/or LIN via lines 142 a, 142 b). Reflux liquid 139-140-141 for the low-pressure column 12 is removed from the high-pressure column 11 here at an intermediate point.

The liquid oxygen 46 that is to be compressed on the inside is drawn off with a lower purity of about 95% from an intermediate point of the low-pressure column 12. In addition, a second, more pure oxygen product 346, 348, 349 can be recovered by internal compression (second oxygen pump 347). Because of the pure oxygen recovery, the air pressure must be, for example, higher than in the previously described embodiments. It is, for example, 6 to 7.5 bar, preferably 6 to 7 bar.

FIG. 4 combines the air circulation of FIG. 2 with the removal of the product of FIG. 3.

The term “actively depressurized” in the specification and claims is meant to be synonymous with the older thermodynamic term “work expansion”. Turbo expanders uncoupled or coupled with compressors are modern work expansion engines.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 10 2007 031 765.6, filed Jul. 7, 2007 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for low-temperature separation of air in a distillation system (20) that has at least one high-pressure column (11) and one low-pressure column (12), in which a branched stream of process air (20, 32, 132) after compression and purification is introduced into the high-pressure column (11), a liquid product stream (42; 46; 346) is removed from the distillation system, brought in the liquid state to an elevated pressure (43; 47; 347) and evaporated or pseudo-evaporated under said elevated pressure by indirect heat exchange (6), and then drawn off as a gaseous product stream (45; 49; 349), all of the process air (1) is compressed in a main air compressor to a first pressure, which is at least equal to the operating pressure of the high-pressure column (11), and then the resultant compressed process air (1) is purified in a purification device, a first partial stream (2, 7) of the resultant purified process air (1) is fed under approximately the first pressure to a first expander (8), actively depressurized therein to approximately the operating pressure of the low-pressure column (12), thereby producing mechanical energy and then introduced (10) into the low-pressure column (12), a second partial stream (3) of the compressed purified process air (1) is compressed in a first secondary compressor (14) to a second pressure that is higher than the first pressure, and at least one portion (17) of the second partial stream (16) downstream from the compression in a second secondary compressor (18) is further compressed to a third pressure, which is higher than the second pressure, fed to the indirect heat exchange (6) for evaporation or pseudo-evaporation of the liquid product stream, and then introduced into the distillation system (20), the second secondary compressor (18) being a cold compressor, and wherein at least a portion of the mechanical energy produced in the active depressurization (8) of the first partial stream (7) drives the second secondary compressor (18).
 2. A process according to claim 1, wherein a third partial stream (4) of the process air is actively depressurized in a second expander (30, 130).
 3. A process according to claim 2, wherein the actively depressurized third partial stream (31) is introduced (10) into the low-pressure column (12).
 4. A process according to claim 2, wherein the actively depressurized third partial stream (131) is introduced (132) into the high-pressure column (11).
 5. A process according to claim 1 wherein all of the process air is compressed to a pressure of more than 1 bar higher than the operating pressure of the high pressure column.
 6. A process according to claim 1 wherein all of the process air is compressed to a pressure of more than 3 bars higher than the operating pressure of the high pressure column. 